The invention relates to a magnetic resonance imaging system including a system of emission antennas for generating one or more RF excitation pulses and a system of receiving antennas for receiving magnetic resonance signals.
A magnetic resonance imaging system including a system of emission antennas and a system of receiving antennas is described in U.S. Pat. No. 5,943,433.
This prior art magnetic resonance imaging system includes receiving antennas in the form of surface coils. The spatial receiving profile of the surface coils is inhomogeneous. This means that the sensitivity of the coils for the reception of the magnetic resonance signals differs as a function of the location where the magnetic resonance signals are generated. The magnetic resonance imaging system performs a special correction so as to ensure that the inhomogeneities of the receiving profile do not cause disturbances in the magnetic resonance image formed from the acquired magnetic resonance signals. The correction is performed using a correction algorithm designed to execute the correction with a high calculation speed.
The magnetic resonance imaging system described in U.S. Pat. No. 5,943,433 requires a comparatively long period of time to receive the magnetic resonance signals.
It is an object of the invention to provide a magnetic resonance imaging system which requires a shorter period of time for receiving the magnetic resonance signals.
This object is achieved by means of a magnetic resonance imaging system in accordance with the invention wherein the system of emission antennas has a spatially inhomogeneous emission profile, and the magnetic resonance imaging system is provided with a reconstruction unit for reconstructing a magnetic resonance image from the magnetic resonance signals while utilizing the emission profile of the system of emission antennas.
In light of the spatially inhomogeneous emission profile of the emission antennas, the intensity and/or the phase of the RF excitation pulse in a given position is dependent on the direction and/or the distance from the emission antenna. It has been found in practice that so-called surface coils are very well suitable for generating a spatially inhomogeneous RF excitation pulse. The spatially inhomogeneous RF excitation of magnetic moments, for example, of the nuclear spins, in the object to be examined produces spatially inhomogeneous magnetic resonance signals. This means that the amplitudes and/or the phases of the magnetic resonance signals vary in space, inter alia due to the spatially inhomogeneous pattern of the RF excitation. Such a spatially inhomogeneous pattern of the RF excitation will be referred to hereinafter as the RF excitation profile.
Furthermore, spatial variations of the amplitudes and the phases of the magnetic resonance signals occur as a result of inhomogeneities in the composition and build-up of the object to be examined. Since the inhomogeneous pattern of the RF excitation is known prior to the reception of the magnetic resonance signals, the variations in the magnetic resonance signals resulting from the inhomogeneity of the RF excitation and the variations that relate to the composition and build-up of the object to be examined can be de-interleaved. The magnetic resonance image can be reconstructed from the magnetic resonance signals on the basis of the RF excitation profile. Since little or no spatial encoding of the magnetic resonance signals by temporary gradient fields is required, less time is required for the acquisition of the magnetic resonance signals. Further, since the RF excitation profile partly or completely takes over the encoding by temporary gradient fields, it is possible to acquire magnetic resonance signals for several parts of the k space simultaneously to a high degree of accuracy.
Preferably, the system of emission antennas generates an RF excitation profile such that the magnetic resonance signals are spatially encoded on the basis of the RF excitation profile. For example, the emission antennas are constructed as surface coils for this purpose. Such surface coils can provide a spatial electric current density that is suitable to generate the desired RF excitation profile.
It is noted that the system of emission antennas encompasses a system comprising a single emission antenna, for example a single emission RF-coil having a spatial emission profile. Also a single receiver antenna, such as a surface coil may be employed as the system of receiver antennas.
In a preferred embodiment of the magnetic resonance imaging system in accordance with the invention, the k space is sub-sampled by the magnetic resonance signals. Only a fraction of the data required for the desired spatial resolution and field of view (FOV) is then acquired. For example, only a part of the necessary lines in the k space is sampled during the acquisition of the magnetic resonance signals e.g., only one half, a quarter or a sixteenth part of the lines in the k space is sampled, every second, every fourth or every sixteenth line, respectively, in the k space then being sampled. The spatial encoding required for the reconstruction of the magnetic resonance image is provided by the RF excitation profile so that the magnetic resonance signals can be received simultaneously to a high degree. For example, a plurality of lines in the k space can be sampled simultaneously.
The reconstruction of the magnetic resonance image utilizes the known SENSE or SMASH techniques. The SENSE technique is known per se from, e.g., the article xe2x80x9cSENSE: sensitivity encoding for fast MRIxe2x80x9d by K. P. Pruesmann et al. in Magnetic Resonance in Medicine 42 (1999) pp. 952-962. The SMASH technique is known per se from, e.g., International Application No. WO 98/21600. The cited publications concerning the SENSE technique and the SMASH technique describe the sub-sampled acquisition of magnetic resonance signals and the formation of the magnetic resonance image on the basis of the coil sensitivity profiles of the surface coils used as receiving antennas. The conventional SENSE technique and the conventional SMASH technique utilize a spatially uniform RF excitation profile.
The reconstruction unit in a preferred embodiment of the magnetic resonance imaging system in accordance with the invention is arranged to reconstruct one or more emission coil images. Such emission coil images are derived from sub-sampled signal sections of magnetic resonance signals. The spatially inhomogeneous RF excitations of individual emission antennas generate the respective signal sections of magnetic resonance signals. The sub-sampling of the signal sections causes so-called aliasing artefacts in the emission coil images. Such aliasing artefacts arise because the field of view in the emission coil image is too small relative to the region from which the signal section originates. It has been found that the SENSE technique enables practically complete or almost complete elimination of the aliasing artefacts in the emission coil images on the basis of the emission profile. A magnetic resonance image of high diagnostic quality can thus be derived from the sub-sampled signal sections.
The magnetic resonance image typically has a spatial resolution which is higher than that of the individual signal sections. Since the signal sections are sub-sampled, that is, the signal sections sample, for example, only every second, fourth or sixteenth line in the k space, the acquisition of the signal sections requires only a small amount of time. It has been found that fast motions in the object to be examined can be reliably tracked. For example, a heart beating at a rate of 150-200/min can be suitably tracked.
In a further embodiment of the magnetic resonance imaging system in accordance with the invention, the sub-sampled magnetic resonance signals are combined on the basis of the emission profile so as to form combination signals. The combination signals have full sampling considering the resolution of the magnetic resonance image. The magnetic resonance image is subsequently reconstructed from the combination signals by means of a technique that is known per se, for example by 2D Fourier transformation.
For example, the spatial distribution of the electric current density through the emission coils is controlled such that the system of emission coils has a spatially sinusoidal profile. Individual so-called spatial harmonic components can thus be excited by the system of emission coils. This means that the RF excitation generated by such a system of emission coils leads to the reception of signal sections of magnetic resonance signals which always relate to a spatial harmonic relating to spatial variations in the object to be examined with a wavelength within a narrow range. The conventional SMASH technique can be applied to such signal sections in order to reconstruct therefrom the magnetic resonance image while utilizing the applied emission profile.
In a further preferred embodiment of the magnetic resonance imaging system in accordance with the invention, the reconstruction of the magnetic resonance image from the magnetic resonance signals also utilizes the receiving profiles of the receiving antennas. An even higher degree of sub-sampling can thus be used. Such a high degree of sub-sampling is achieved by applying sub-sampling during the excitation as well as during the acquisition of the magnetic resonance signals. Both the emission profiles and the receiving profiles provide a part of the spatial encoding of the magnetic resonance signals, so that the spatial encoding by gradient fields is substantially reduced. As a result, it is sufficient to sample the k space with a very low density. In that cases magnetic resonance signals are required, for example, for very few lines at a rather large distance from one another in the k space. Magnetic resonance signals can thus be acquired with a very simple, brief RF excitation and gradient pulse sequences wherefrom a magnetic resonance image having a high spatial resolution and a high contrast resolution is reconstructed nevertheless. For example, emission/receiving coil images can be reconstructed from respective signal sections of magnetic resonance signals that have been generated by means of respective emission coils and received by means of respective receiving coils.
In view of the high degree of sub-sampling of the signal sections, so-called aliasing artefacts occur to a high degree in the emission/receiving coil images. These artefacts are caused by the fact that individual pixels in the emission/receiving coil images contain contributions from different positions in the object to be examined. Utilizing the emission profiles of the emission coils and the receiving profiles of the receiving antenna, the magnetic resonance image can be derived from the emission/receiving coil images while decomposing the pixels of the emission/receiving coil images into the contributions from different positions within the object. Such decomposition can be performed, for example by means of the SENSE method that is known per se.
It is also possible to perform an interpolation in the k space from the signal sections of sub-sampled magnetic resonance signals generated by the RF excitations by respective emission coils and acquired by means of respective receiving coils, that is, an interpolation on the basis of weighting factors, so as to execute full sampling of (a part of) the k space. The weighting factors in the interpolation are dependent on the emission profiles and the receiving profiles.
These and other aspects of the invention will be described in detail hereinafter, by way of example, on the basis of the following embodiments and with reference to the accompanying drawing.